Microporous polytetrafluoroethylene and the production thereof was first described in U.S. Pat. Nos. 3,953,566 and 4,187,390. The material consists solely of high-molecular emulsion polymerisate of PTFE (hereinafter referred to as PTFE) which is paste-extruded and then expanded. U.S. Pat. No. 3,953,566 describes the production of an expanded microporous PTFE with a stable structure, in which coagulated dispersions are used as starting materials. These products have found a wide range of applications due to their chemical and physical stability.
The state of the art also includes modifications of PTFE having certain chemical and physical properties. U.S. Pat. No. 5,098,625, for instance, describes a process for the production of porous PTFE-membranes of increased density, improved flexibility and reduced cold flow which are also made from coagulated dispersions. This type of PTFE, however, has less tensile strength than the unmodified PTFE.
EP 418155 proposes to partially sinter high-molecular PTFE and expand it afterwards to improve thermal stability and dimensional stability and to increase porosity.
Furthermore U.S. Pat. No. 5,064,593 describes a multilayer PTFE-membrane which comprises a small-pored filter layer made from a non-fibril forming PTFE fine powder and an open-pored supporting layer made from high-molecular PTFE. Each layer must be produced separately and the separate layers are linked with each other during processing.
U.S. Pat. No. 5,102,921 describes a PTFE-material of high porosity with a large pore diameter, which is obtained by a mixture of a high-molecular and a low-molecular PTFE fine powder. These products, however, have a low mechanical strength.
In this connection, U.S. Pat. No. 5,087,641 suggests a process in which a microporous membrane is subsequently modified with a dispersion of a sintered or irradiated PTFE material. The additional component is incorporated into the pore interstices, which creates a certain reinforcing effect.
A modification of the properties of porous PTFE material by admixtures of low-molecular PTFE is desirable in particular with a view to the reutilization of polymerisate scrap.
Generally speaking high-molecular emulsion polymerisates of PTFE can form fibril structures during the paste extrusion and expansion steps. Low-molecular PTFE-types, however, cannot be expanded and thus will not form any fibril structures.
Experts know that irradiated PTFE generally have reduced mechanical strength.
There is a need for a material of porous polytetrafluoroethylene which can be produced by incorporating perfluorinated non-expandable polymers into high-molecular PTFE-material without compromising the desirable mechanical properties such as tear resistance, tensile strength and break elongation.